The present invention relates to a wrought material of an aluminum alloy to be anodized for facing structures such as buildings and a process for making the same and, more particularly, to a wrought material of an aluminum alloy to be anodized into the so-called "dense gray", i.e., a gray or dark gray color and a process for making the same.
Generally speaking, major aluminum alloys to be anodized for facing buildings or the like in the prior art are alloys JIS (which is the abbreviation of "Japanese Industrial Standards") Nos. 1100, 1050 and 5005, which have colors represented by a light gray through an anodization in a sulfuric acid bath, by a brown through a natural coloring anodization or through a coloring by the so-called "Asada method". In recent years, however, from the standpoint of seeking for apparent depth of buildings, there is an intense demand for an aluminum alloy assuming a color of the dense gray, i.e., a gray or dark gray after the anodization.
As the building aluminum alloy materials assuming the aforementioned gray or dark gray color after the anodization, there has been partially used in Al-Si alloy JIS No. 4343 or its improved alloy.
However, this Al-Si alloy JIS No. 4343 or its improved alloy can assume a color of dense gray after the anodization but is susceptible to the influence of heat. As a result. The color is liable to fluctuate not only among the production lots but also in one lot. This makes it extremely difficult to produce an alloy plate which can stably assume the dense gray in an identical tone. On the other hand, the Al-Si alloy is defective in that it has a lower anticorrosion after the anodization than those of the above-specified alloys JIS Nos. 1100 and 5005. This raises another problem as the aluminum alloy to be used for facing the buildings.
Incidentally, it is known that a pattern called the "fir-tree structure" frequently appears in the section of an alloy ingot which will separate an intermetallic compound of Al-Fe, such as the alloy JIS No. 1100, 1050 or 5005. In this fir-tree structure, the ingot has its internal region assuming a relatively dark gray and its external region near the surface assuming a relatively light gray in its section when it is anodized. Thus, the fir-tree structure is named so because the boundary between the darker gray portion (i.e., the internal region) and the lighter gray portion (i.e., the external region) appears as it were a fir-tree in the longitudinal section of the ingot. It is also known that the fir-tree structure is caused by the difference in the kinds of crystallizing Al-Fe compounds depending upon the portions of the ingot. Roughly speaking, crystallizing in the ingot the intermetallic compounds Al .sub.m Fe, Al.sub.3 Fe and Al.sub.6 Fe, which have such different electrochemical properties that the phases of Al.sub.m Fe and Al.sub.3 Fe are oxidized during the anodization to exist as oxides in the oxidized film whereas the phase of Al.sub.6 Fe is not oxidized to exist as the metallic phase in the film. This Al.sub.6 Fe phase left unoxidized, if any in the film, will absorb an incident light to assume a darker gray than the other Al.sub.3 Fe and Al.sub.m Fe phases. Generally speaking, the Al.sub.m Fe phase is present mainly in the external region of the fir-tree structure whereas the Al.sub.6 Fe and Al.sub.3 Fe are present in the internal region so that this internal region containing the Al .sub.6 Fe phase exhibits a darker gray than the external region composed mainly of the Al.sub.m Fe phases, as is known in the art.
It is, therefore, conceivable to make an aluminum alloy sheet which can assume a dense gray, i.e., a gray or dark gray, after anodized, even if it is not made of the aforementioned Al-Si alloy, if the internal region of the fir-tree structure is so enlarged as to construct the ingot in its entirely of the structure of the internal region of the fir-tree structure thereby to make an ingot substantially having no fir-tree structure.
In Japanese Patent Publication No. 58 - 26431, on the other hand, we have already proposed a composition for enlarging the internal region of the fir-tree structure of aluminum alloys of Al-Fe-Si-Mg to make a structure wholly of the internal region. According to the invention proposed, it is possible to make an alloy sheet which is anodized into a color of the gray or dark gray, and this alloy can enjoy more excellent anticorrosion than the aforementioned Al-Si alloy. Despite this possibility, however, the fact is that the color of the gray or dark gray is not always stabilized if the whole ingot is constructed of the structure of the internal region of the fir-tree structure according to that proposal. Since, not only the Al.sub.6 Fe phase assumes the dark gray but also the Al.sub.3 Fe phase will crystallize into the internal region of the fir-tree structure, as has been described hereinbefore, even the structure of the internal region will cause a change in the gray tone if the ratio between the Al.sub.6 Fe and Al.sub.3 Fe phases changes. This color may sometimes fluctuate depending especially upon the casting and hot working conditions.
We therefore have repeated experiments and researches so as to develop both an aluminum alloy of Al-Fe-Si-Mg, which can be stably anodized to assume an identical color of gray or dark gray, and a process for making the same. As a result, it is found, as has already been proposed in Japanese Patent Laid-Open No. 61 - 110741 (corresponding to Japanese Patent Application No. 59 - 231849) that it is necessary for stably assuming a constant color of gray or dark gray to have the Al.sub.6 Fe phase occupy 70% or more of the total intermetallic compound of Al-Fe in the portion of the cast ingot, which will finally form the surface of a rolled sheet, i.e., the (skin) portion of 50 mm depth from the surface of the ingot, by strictly regulating the ratio of Fe and Si in relation to Mg and by properly setting the ingot casting conditions.
According to the above-specified proposal of Japanese Patent Laid-Open No. 61 - 110741, the Al.sub.6 Fe phase can crystallize relatively stably to considerably stabilize the gray or dark gray color. It is, however found, in the case of the composition of the Al-Fe-Si-Mg proposed, that the mixing ratio of the two Al.sub.6 Fe and Al.sub.3 Fe phases will still fluctuate depending upon the casting conditions so that the anodized color will fluctuate in the lot and among the lots of the final rolled sheets.